It's a funny experience, interviewing an interviewer. As co-host of Channel 4's Popworld, Simon Amstell acquired a cult following by asking pop stars impertinent questions. Even when letting them plug their new single, he made them work for it. He once asked a flustered Rachel Stevens to "tell me about your video in a way which will make me go, OK, let's have a look at it then".

I ask him to tell me about his Edinburgh festival show in a way that will make me go, OK, let's go and see it then. "Do you want the list of things?" he says, delighted. "I made a list earlier. It's about compassion, integrity, divorce, breeding, ethical and moral issues, money, guilt and, er, social awkwardness. And some other things I've forgotten. You're still not convinced are you?" He emits a shrill, embarrassed laugh. "OK, it's about me. If you like me, you'll love the show." That laugh again.

Amstell started doing stand-up at 14 and has been working the comedy circuit on and off ever since. At last year's Edinburgh festival, he hosted a kind of chatshow - but only since he left Popworld in April has he felt ready to mount a proper, one-hour solo performance. "I think I've figured out who I am now as a stand-up comedian. For the past five years, I was getting away with it a bit. I want it to be as personal and true as it can possibly be. If a joke isn't real, I have to take it out."

Amstell is obsessed with honesty. The first thing he does when he parks his gangly frame on a sofa in his manager's office is apologise: "I have problems answering even simple questions." He didn't want to do any promotional interviews ("I wanted to be one of those reclusive geniuses") until the promoter gently pointed out that he needed to sell some tickets. He does get in a tangle over how he comes across. A simple inquiry about what music he likes elicits such a tortured response that I give up. "If I said Radiohead it would just sound awful," he says. Even when Amstell's telling the truth, he's concerned about appearing false.

It's hard to square this fidgeting, giggling worrier with the unflappable Popworld host who weathered the thunderous ire of Cheryl Tweedy and the arctic disdain of the Strokes. He says it's simple. "I've always felt safe if there's a camera there. You can say or do anything if there's someone filming it. It's all just make-believe."

When Popworld started five years ago, Amstell was 21 and recently sacked from Nickelodeon for being, he says proudly, "sarcastic and mean to children". The show struggled to finds its voice; Amstell and his 16-year-old co-host Miquita Oliver did not instantly bond. Gradually, it settled on a tone somewhere between Smash Hits and a good chatshow, a mixture of puckish surrealism and blunt questions delivered with disarming charm. "I'd watch Planet Pop and The O-Zone and I'd always be frustrated," he says. "The biggest and most important - well, not important because it's pop and it's nonsense - but the most interesting question wouldn't be asked. We felt like we had a duty to ask whatever was on people's minds."

The Popworld approach effectively divided guests into three categories: the good (McFly), the bad (the "vile" Girls Aloud) and the dull (Ronan Keating). Amstell thinks the humour came from the reactions rather than the lines. Among the clips currently circulating on YouTube, you can see the Kooks' Luke Pritchard throwing a hissy fit over the subject of stage school and Britney Spears flummoxed by the question: "Have you ever licked a battery?" (She hadn't.)

Popworld's guiding principle wasn't contempt for pop stars so much as cheerful disregard. Here, at last, were two pop presenters with absolutely no desire to befriend their interviewees. "To see people on TV pretending to like one another is horrible," says Amstell. "Even if I was hanging out with Razorlight - not that I am - I wouldn't want people to know. It's horrible. People getting on! That's not funny. I like people being tense with each other."

Amstell and Oliver left Popworld in April; the last episode featured Daniel Bedingfield, dressed as God, banishing them to hell. "We just thought, 'It's been five years. What else is there to say? What else is there to do?'" Has he seen the current hosts (Alex Zane and Alexa Chung) in action? "I watched the first two weeks to, er ... check."

To check they weren't funnier? "To check they weren't funnier!" he hoots. "I stopped after that. It would have been tragic to keep watching."

Would he have been hurt if they had been funnier? "I was a bit worried. They did a good trailer. I thought, 'Hang on, this is a bit cool. What if they're really good?' It was our show so if two other people had come in and nobody had noticed any difference that would have been a bit upsetting."

Amstell decided long ago he didn't want to be a "rent-a-music-presenter", a name on a list. He says Popworld offered so much creative freedom that virtually anything else would be a disappointment. His resolve was hardened last year by hosting the short-lived entertainment programme The Morning After Show, the mention of which causes him to fold with embarrassment. "You know when people say they put their head in their hands? I thought that was just an expression; then I started doing it. It taught me you have to be in control from the beginning, otherwise it's just a nightmare."

Talking about the early days of Popworld, Martin Cunning, of the show's production company, described the young Amstell as a "twisted 21-year-old gay geek sitting in the Essex suburbs bored out of his mind". I ask Amstell if he thinks that's a fair summary. "Pretty true. I was 21, I wasn't quite gay yet, definitely a geek, definitely in Essex." Not gay yet? "Yeah. I mean I was but I hadn't come out to myself or anyone else. I was in denial and I thought if I didn't tell anyone that it wasn't real."

Two months into the show, Amstell came out. "No one believed me for a week, so I had to keep going on about it. I said it in the studio one day and only the runner heard me." He used to feature a lot of gay jokes in his stand-up but he's dropped them. "It feels really old. No one cares any more. There are so many homos on TV. It's why Matt Lucas ended up doing the only gay in the village. It was a twisted coming-out story because the old one is so boring."

Amstell got an early taste for applause in school shows. "Someone else had a funny line and I thought, 'I want the funny line.'" He became a chatshow junkie, watching Oprah, Ruby Wax and Jonathan Ross. He'd like to host a chat show one day, but not yet. "I'm too young. I felt like a stupid child [on Popworld] talking to Debbie Harry about Andy Warhol." It's true; even at 26, he's so boyish a barman might refuse him service.

Did he want to be famous? "I did but as soon as I got recognised for the first time I realised that there was no buzz. Nothing happened." So how well-known is he? "It's a fairly odd level of fame: one person will recognise me and the person next to them will be really loud about not knowing."

In stand-up, he says, fame's irrelevant anyway. "You're either funny or you're not. If Mariah Carey, as famous as she is, went to the Comedy Store, after five minutes people would start heckling. One of the most important things is knowing why you're there. Knowing the material is worth someone coming to see you for an hour and that what you're saying is enough to keep them interested. Otherwise it's embarrassing, isn't it?"

· Simon Amstell is at the Queen Dome, Edinburgh, from tonight until August 28. Box office: 0131-556 6550